Variable stiffness coil for vasoocclusive devices

ABSTRACT

The variable stiffness vasoocclusive coil is given variable stiffness along the length of the coil by selectively heat treating certain segments of a primary or secondary coil. The primary coil can be selectively heat treated to form soft or deformable segments along the length of the coil, and can then be shaped into a secondary shape that is set by a heat treatment process. Distal regions of the coil can also be heat treated to make the distal ends of the coil softer, more deformable, or less traumatic.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] This invention relates generally to implantable devices forinterventional therapeutic treatment or vascular surgery, and moreparticularly concerns a variable stiffness vasoocclusive coil thatexhibits variable stiffness along the length of the coil.

[0003] 2. Description of Related Art

[0004] The art and science of interventional therapy and surgery hascontinually progressed towards treatment of internal defects anddiseases by use of ever smaller incisions or access through thevasculature or body openings in order to reduce the trauma to tissuesurrounding the treatment site. One important aspect of such treatmentsinvolves the use of catheters to place therapeutic devices at atreatment site by access through the vasculature. Examples of suchprocedures include transluminal angioplasty, placement of stents toreinforce the walls of a blood vessel or the like and the use ofvasoocclusive devices to treat defects in the vasculature. There is aconstant drive by those practicing in the art to develop new and morecapable systems for such applications. When coupled with developments inbiological treatment capabilities, there is an expanding need fortechnologies that enhance the performance of interventional therapeuticdevices and systems.

[0005] One specific field of interventional therapy that has been ableto advantageously use recent developments in technology is the treatmentof neurovascular defects. More specifically, as smaller and more capablestructures and materials have been developed, treatment of vasculardefects in the human brain which were previously untreatable orrepresented unacceptable risks via conventional surgery have becomeamenable to treatment. One type of non-surgical therapy that has becomeadvantageous for the treatment of defects in the neurovasculature hasbeen the placement by way of a catheter of vasoocclusive devices in adamaged portion of a vein or artery.

[0006] Vasoocclusion devices are therapeutic devices that are placedwithin the vasculature of the human body, typically via a catheter,either to block the flow of blood through a vessel making up thatportion of the vasculature through the formation of an embolus or toform such an embolus within an aneurysm stemming from the vessel. Thevasoocclusive devices can take a variety of configurations, and aregenerally formed of one or more elements that are larger in the deployedconfiguration than when they are within the delivery catheter prior toplacement. One widely used vasoocclusive device is a helical wire coilhaving a deployed configuration which may be dimensioned to engage thewalls of the vessels. One anatomically shaped vasoocclusive device thatforms itself into a shape of an anatomical cavity such as an aneurysmand is made of a pre-formed strand of flexible material that can be anickel-titanium alloy is known from U.S. Pat. No. 5,645,558, which isspecifically incorporated by reference herein. That vasoocclusive devicecomprises one or more vasoocclusive members wound to form a generallyspherical or ovoid shape in a relaxed state. The vasoocclusive memberscan be a helically wound coil or a co-woven braid formed of abiocompatible material, and the device is sized and shaped to fit withina vascular cavity or vesicle, such as for treatment of an aneurysm orfistula. The vasoocclusive member can be first helically wound orbraided in a generally linear fashion, and is then wound around anappropriately shaped mandrel or form, and heat treated to retain theshape after removal from the heating form. Radiopacity can be providedin the vasoocclusive members by weaving in synthetic or natural fibersfilled with powdered radiopaque material, such as powdered tantalum,powdered tungsten, powdered bismuth oxide or powdered barium sulfate,which can potentially be released during vascular surgery.

[0007] The delivery of such vasoocclusive devices can be accomplished bya variety of means, including via a catheter in which the device ispushed through the catheter by a pusher to deploy the device. Thevasoocclusive devices, which can have a primary shape of a coil of wirethat is then formed into a more complex secondary shape, can be producedin such a way that they will pass through the lumen of a catheter in alinear shape and take on a complex shape as originally formed afterbeing deployed into the area of interest, such as an aneurysm. A varietyof detachment mechanisms to release the device from a pusher have beendeveloped and are known in the art.

[0008] For treatment of areas of the small diameter vasculature such asa small artery or vein in the brain, for example, and for treatment ofaneurysms and the like, micro-coils formed of very small diameter wireare used in order to restrict, reinforce, or to occlude such smalldiameter areas of the vasculature. A variety of materials have beensuggested for use in such micro-coils, including nickel-titanium alloys,copper, stainless steel, platinum, tungsten, various plastics or thelike, each of which offers certain benefits in various applications.Nickel-titanium alloys are particularly advantageous for the fabricationof such micro coils, in that they can have super-elastic or shape memoryproperties, and thus can be manufactured to easily fit into a linearportion of a catheter, but attain their originally formed, more complexshape when deployed.

[0009] One known technique for filling wide neck aneurysms involvesbreaking a coil or permanently deforming a coil within a vesselutilizing a balloon. However, substantial risks to a patient areinvolved in such a procedure, and a coil which has soft or deformablesegments may offer less risk to a patient. As a coil is inserted intothe aneurysm, the coil deforms and sets it shape, but over time a coilwill typically assume its original shape, which is unlikely tocorrespond to the shape of the vessel being filled. Filling of a varietyof types of aneurysms of various sizes and shapes may benefit by use ofa variable stiffness coil that can deform more readily at certainpredetermined sections. As such a variable stiffness coil is insertedinto the aneurysm, the coil will deform to conform to the shape and sizeof the vessel being filled, and will set its shape, but unlike a helicalcoil which over time takes on its original shape, a variable stiffness,deformable coil will permanently deform in a random configuration, tothereby fill an aneurysm more evenly and completely over long periods oftime.

[0010] A variable cross-section conical vasoocclusive coil is known thatcan achieve variations in stiffness of the coil by variation of thediameter in different regions of the coil or variations in thecomposition of the coil. Methods are also known for construction of astent with a varying radial spring force, by heat treatments, by varyingthe stent frame thickness, selectively machining stent ring frames,using different alloys of the ring frames, and varying the Austenitefinish transformation temperature (Af) of a shape memory alloy such asNitinol. A guide wire is also known that is formed from one or more heatactivated memory alloys, with intermediate portions that are selectivelyannealed to have variously curved shapes while the remainder of the wireremains straight when heated, and a stent is known that has U-shapedloop portions that are provided with greater flexibility by selectiveannealing to impart selective degrees of hardness to different portions.

[0011] It would be desirable to provide an vasoocclusive coil withprimary and secondary shapes with variable stiffness along the length ofthe coil that can permanently deform in a random configuration that willpermanently deform in a random configuration in order to fill ananeurysm more evenly and completely over long periods of time. Thepresent invention meets these and other needs.

SUMMARY OF THE INVENTION

[0012] Briefly, and in general terms, the present invention provides fora variable stiffness vasoocclusive coil that exhibits variable stiffnessalong the length of the coil. Variable stiffness is accomplished byselectively heat treating certain segments of a primary or secondarycoil. The primary coil can be selectively heat treated to form soft ordeformable segments along the length of the coil, and can then be shapedinto a secondary shape that is set by a heat treatment process. Asecondary coil such as a three dimensional coil can be produced withvariable stiffness through a selective heating of localized segments ofthe coil. Distal regions of the coil can also be heat treated to makethe distal ends of the coil softer, more deformable, or less traumatic.Upon deployment, the coil will take on its preformed three dimensionalshape, and will deform in a random three-dimensional shape to conform tothe shape of the vessel or malformation into which the coil isintroduced. The variable stiffness coil is advantageously formed of ashape memory metal, and variable stiffness can be achieved through agingof desired segments of the shape memory metal coil to raise the parentphase or Austenite phase finish temperature, thus making the treatedsegments of shape memory metal softer and more flexible.

[0013] The invention accordingly provides for an occlusive device foruse in interventional therapy and vascular surgery adapted to beinserted into a portion of a vasculature for occluding the portion ofthe vasculature of a patient. The occlusive device comprises a variablestiffness coil formed from one or more flexible strands of a shapememory metal having a primary coil configuration, the coil having aplurality of segments heat treated to cause the plurality of segments tohave reduced stiffness. In one presently preferred embodiment, thevariable stiffness coil has an expanded secondary coil configurationwith a secondary three dimensional shape, such as a spherical or helicalshape. In a preferred aspect, the flexible strand comprises asuper-elastic material, which can be a shape memory metal such as anickel titanium alloy. The shape memory nickel-titanium alloy ispreferably heat treated such that the alloy is highly flexible at atemperature appropriate for introduction into the vasculature via acatheter, and after placement, the device will take on a shape designedto optimize the therapeutic purposes desired for the device.

[0014] The invention also provides for a method for making a variablestiffness occlusive coil for use in interventional therapy and vascularsurgery adapted to be inserted into a portion of a vasculature foroccluding the portion of the vasculature of a patient, comprising thesteps of providing a coil formed from one or more flexible strands of ashape memory metal, the coil having a primary coil configuration and aninitial stiffness; and heat treating a plurality of segments of the coilto cause the plurality of segments to have reduced stiffness. In onepresently preferred embodiment, the step of providing a coil comprisesheating the coil in a desired three dimensional configuration to set thethree dimensional shape. In a preferred aspect of the method of theinvention, the shape memory metal has an Austenite phase finishtemperature, and the step of heating the coil comprises heating the coilat about 475° C. to 525° C. for about 1 to 20 minutes to set theAustenite phase finish temperature of the coil to about −5° C. to 10° C.The step of heat treating the coil can be accomplished by artificiallyaging a plurality of segments of the coil to raise the Austenite phasefinish temperature to about 35° C. to 50° C., such as by heating aplurality of segments of the coil to a temperature of about 400° C. fora period of about 5 seconds to 30 minutes.

[0015] These and other aspects and advantages of the invention willbecome apparent from the following detailed description and theaccompanying drawings, which illustrate by way of example the featuresof the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016]FIG. 1 is a primary helical vasoocclusive coil showing areas ofheat treatment according to the invention.

[0017]FIG. 2 is a secondary helical vasoocclusive structure formed usingthe primary helical coil of FIG. 1.

[0018]FIG. 3 is a secondary spherical vasoocclusive structure formedusing the primary helical coil of FIG. 1.

[0019]FIG. 4 is a graph illustrating the reduction in stiffness of ashape memory coil by heat treatment according to the principles of theinvention.

[0020]FIG. 5 is a schematic diagram of an apparatus for applying heat tosegments of a vasoocclusive coil to form a variable stiffnessvasoocclusive coil according to the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0021] Modern techniques for filling wide neck aneurysms typicallyinvolve breaking a coil or permanently deforming a coil within a vesselutilizing a balloon, with attendant substantial risks to a patient, anda coil which has soft or deformable segments may offer less risk to apatient. While modem vasoocclusive coils deform and set their shape whenthey are introduced into a vessel, over time such coils will typicallyassume their original shape rather than to the shape of the vessel beingfilled. Filling of a variety of types of aneurysms of various sizes andshapes may benefit by use of a variable stiffness coil that can deformmore readily at certain predetermined sections to fill an aneurysm moreevenly and completely over long periods of time.

[0022] As is illustrated in the drawings, the invention is embodied inan occlusive device for use in interventional therapy and vascularsurgery adapted to be inserted into a portion of a vasculature foroccluding a selected portion of the vasculature of a patient. In apresently preferred embodiment of the invention illustrated in FIG. 1,the occlusive device 10 is made from a strand of wire of approximately0.001 inch to approximately 0.006 inch in diameter and comprises a coil12 formed from one or more flexible strands of a super-elastic, shapememory metal such as nickel-titanium alloy, for example. While the abovestated range of diameters is presently known to be compatible with theinvention, larger or smaller diameters may be useful for particularapplications. The occlusive device typically has at least a primary coilconfiguration illustrated in FIG. 1, with a plurality of segments 14being heat treated to cause the plurality of segments to have reducedstiffness.

[0023] In one presently preferred embodiment shown in FIG. 2, thevariable stiffness coil has an expanded secondary coil configurationwith a secondary helical three dimensional shape 16, with localized heattreated segments 18, although the variable stiffness coil can also havean expanded secondary coil configuration with a secondary sphericalthree dimensional shape 20, with localized heat treated segments 22, asillustrated in FIG. 3. The shape memory metal is preferably heat treatedto be highly flexible at a temperature appropriate for introduction intothe vasculature via a catheter, and such that after placement, thedevice will take on a shape designed to optimize the therapeuticpurposes desired for the device.

[0024] The invention also provides for a method for making the variablestiffness occlusive coil. In a presently preferred embodiment, thevariable stiffness occlusive coil can be formed from a coil 12 of one ormore flexible strands of a superelastic shape memory metal. The coilpreferably has at least a primary coil configuration and an initialstiffness, as is illustrated in FIG. 4, representing the change instiffness of a heat treated segment of such a coil by application ofheat to the segment, such as by the apparatus shown in FIG. 5. Variablestiffness of the heat treated segment can be achieved through artificialaging of the shape memory metal, such as Nitinol. The shape memorybehavior of the shape memory metal can be modified by artificial agingof the material by heat treatment affecting the Austenitictransformation temperatures. When a shape memory alloy such as nickeltitanium alloy is deformed, and then heated to recover its originalparent or Austenite shape, the original shape corresponds to the shapeof the alloy in the relatively high temperature range of the parentphase. Once the Austenite phase finish temperature (Af) is reached, thenickel titanium alloy becomes stiffened. However, artificial aging ofthe nickel titanium alloy can raise the Af temperature, thus making thematerial act softer at higher temperatures. The coil is preferablyinitially heated in a desired three dimensional configuration to set thethree dimensional by heating the coil, such as in a salt pot, at about475° C. to 525° C. for about 1 to 20 minutes to set the Austenite phasefinish temperature of the coil to about −5° C. to 10° C. As isillustrated in FIG. 5, heat treating of a segment of the coil will causethe segment to have reduced stiffness, such as by artificially aging thesegment of the coil to raise the Austenite phase finish temperature toabout 35° C. to 50° C., by heating a plurality of segments of the coilto a temperature of about 400° C. for a period of about 5 seconds to 30minutes. This can be accomplished by placing the primary or secondaryshape coil in a heated air box 26 supplying hot air from a source ofheated air (not shown). The air box, typically made of brass, forexample, has a channel 30 in which the coil can be placed to expose thecoil 28 to a flow of hot air from a port 32 that is typically 0.020inches to 0.500 inches in diameter, conveyed to the port through aconduit 34 that extends through the air box. In this manner, localizedheating can be provided to desired portions of the coil, at controlledtemperatures for prescribed periods of time. Alternatively, heating ofsegments of the coil can be achieved by other means, such as by a laser,or by electrical heating, or other common types of heating elements.

[0025] It will be apparent from the foregoing that while particularforms of the invention have been illustrated and described, variousmodifications can be made without departing from the spirit and scope ofthe invention. Accordingly, it is not intended that the invention belimited, except as by the appended claims.

What is claimed is:
 1. An occlusive device for use in interventionaltherapy and vascular surgery adapted to be inserted into a portion of avasculature for occluding the portion of the vasculature of a patient,comprising: a variable stiffness coil formed from at least one flexiblestrand of a flexible material having a primary coil configuration, saidcoil having a plurality of segments heat treated to cause the pluralityof segments to have reduced stiffness.
 2. The occlusive device of claim1, wherein said variable stiffness coil has an expanded secondary coilconfiguration with a secondary three dimensional shape.
 3. The occlusivedevice of claim 2, wherein said secondary three dimensional shape isgenerally spherical.
 4. The occlusive device of claim 2, wherein saidsecondary three dimensional shape is generally helical.
 5. The occlusivedevice of claim 1, wherein said at least one flexible strand comprises asuper-elastic material.
 6. The occlusive device of claim 5, wherein saidsuper-elastic material comprises a nickel-titanium alloy.
 7. Theocclusive device of claim 6, wherein said nickel-titanium alloy is heattreated such that the alloy is highly flexible at a temperatureappropriate for introduction into the vasculature via a catheter, andafter placement, the device will take on a shape designed to optimizethe therapeutic purposes desired for the device.
 8. The occlusive deviceof claim 1, wherein said at least one strand of flexible material is astrand of shape memory metal alloy.
 9. The occlusive device of claim 8,wherein said shape metal alloy is a nickel-titanium alloy.
 10. A methodfor making a variable stiffness occlusive coil for use in interventionaltherapy and vascular surgery adapted to be inserted into a portion of avasculature for occluding the portion of the vasculature of a patient,comprising the steps of: providing a coil formed from at least oneflexible strand of a shape memory metal, said coil having a primary coilconfiguration and an initial stiffness; and heat treating a plurality ofsegments of said coil to cause said plurality of segments to havereduced stiffness.
 11. The method of claim 10, wherein said step ofproviding a coil comprises heating said coil in a desired threedimensional configuration to set said three dimensional configuration.12. The method of claim 11, wherein said shape memory metal has anAustenite phase finish temperature, and said step of heating said coilcomprises heating said coil at about 475° C. to 525° C. for about 1 to20 minutes to set the Austenite phase finish temperature of the coil toabout −5° C. to 10° C.
 13. The method of claim 10, wherein said step ofheat treating comprises artificially aging a plurality of segments ofsaid coil to raise the Austenite phase finish temperature to about 35°C. to 50° C.
 14. The method of claim 10, wherein said step of heattreating comprises heating a plurality of segments of said coil to atemperature of about 375° C. to 425° C. for a period of about 5 secondsto 30 minutes.
 15. The method of claim 10, wherein said step of heattreating comprises heat treating a distal segment of said coil to causesaid distal segment to have reduced stiffness.